It has long been known to acoustically log open wellbores to determine the velocities of compressional waves and shear waves traveling through rock formations located in the wellbore region. Logging devices have been used for this purpose which normally comprise one or more transmitters and one or more receivers disposed at preselected distances in the wellbore.
By timing the travel of compressional waves, shear waves, and/or tube waves between the transmitters and receivers, it is normally possible to determine the nature of surrounding rock formations. In logging loosely consolidated formations, however, it is often difficult to distinguish between compressional, shear, tube and secondary waves which may comprise portions of a wave train arriving at a given receiver. Remotely spaced, multiple receivers have been used to aid in distinguishing between arriving wave fronts and from noise in the system. Multiple receivers permit the recognition of similar wave patterns and wave fronts which are received at each successive receiver. Since travel time differentials increase with increasing distance from the transmitter source, wave fronts and patterns which are closely spaced at proximate receiver locations will separate by the time of their receipt at remote receiver locations.
Various signal timing and wave front analysis methods have also been used for distinguishing between wave fronts received at a given receiver. Most of these methods involve timing circuits which anticipate the receipt of, and facilitate the collection of, such wave front information. For descriptions of various logging techniques for collecting and analyzing compression wave, shear wave, tube wave, and secondary wave data, reference may be made to U.S. Pat. Nos. 3,333,238 (Caldwell), 3,362,011 (Zemanek, J. R.) and U.S. No. Reissue 24,446 (Summers).
In the design of logging tools, various types of transmitters, such as, piezoelectric or magnostrictive transmitters, have been suggested for creating acoustic logging signals. For conventional logging operations, most such transmitters have been centrally located in the borehole, and have been adapted to generate sound which is radiated in a multidirectional (360.degree.) pattern from the transmitter to adjacent wellbore surfaces. Such transmitters are well suited for creating compressional waves in surrounding rock and sand formations.
Since compressional waves travel faster than those shear, tube or secondary waves which may also be produced by a multidirectional transmitter, calculation of compressional wave velocity is accomplished by presuming the the first arriving wave front or wave pattern is that of a compressional wave. In loosely consolidated formations, subsequent arrivals of shear waves are difficult to distinguish. In such formations, multidirectional transmitters tend to generate compressional waves of much greater amplitudes than any shear waves also produced thereby. Recognition of shear wave arrivals, is thus particularly difficult. Compressional and shear waves propagate along critical angle refraction paths. If the shear wave velocity in unconsolidated formations in less than the velocity in the wellbore liquid (e.g. drilling fluid, water or oil), then there is no critical angle refraction path for the shear wave and no shear wave is generated at the wellbore liquid interface.
Recently, attention has been directed to developing transmitters which are particularly suited to shear wave logging. Such transmitters, generally termed bender-type transducers, attempt to achieve a single point-force application of sound energy to the borehole wall. The theory behind point-force transmitters, as generally outlined in "A New Method for Shear Wave Logging", by Choro Kitsunezaki, Oyo Technical Note RP-4101, Oyo Corporation, Urawa Saitama 336 Japan (October, 1978) is that they are capable of directly generating shear waves. Conventional multidirectional transmitters are said to be capable only of indirectly creating shear waves in accordance with a critical angle refraction path process. Accordingly, point force type transmitters produce shear waves of substantially higher amplitudes than heretofore possible with conventional multidirectional compressional wave transmitters. Accordingly, formations, such as loosely consolidated sand, which do not propagate shear waves in sufficient amplitudes to permit definitive detection or unconsolidated sand which do not permit propagation of shear waves using conventional compressional wave transducers, may now be shear wave logged with these shear wave logging systems. Oyo Technical Notes RP-4105, entitled "Development of a Suspension Type S-Wave Log System", by Kimio Ogura (November 1979) and RP-4125, entitled "Development of the Suspension S-Wave Logging System (Report No. 2)", by Kimio Ogura, et al (November 1980) provide additional information relating to shear wave logging systems.